1. Field of the Invention
The invention relates to a method of assembling successively fed paper sheets subjected to a printing treatment, and groupwise delivery thereof to a following station for further treatment. The invention also relates to an apparatus for performing a method of this kind.
2. Description of the Background Art
In printing apparatus, such as copying machines and printers, successive paper sheets provided with different images by the printing apparatus are assembled, for example to form manageable bundles, e.g. by means of a staple delivered by a stapling machine included in the printing apparatus. It is also possible that the paper sheets will be joined together somewhere else, but the paper sheets must be available in packets for the purpose. In all these cases, the continuous sequential processing of the paper sheets is converted into a group-wise (end) processing. Appropriate steps must be taken to ensure that the continuous actions may take place unobstructedly during the group processing, e.g. the alignment of a stack of paper sheets and the passing of a staple therethrough.
Buffer systems suitable for this purpose are known, for example, from U.S. Pat. No. 5,289,251 and European patent application 0 613 846. Both of these documents refer to two pairs of transport rollers spaced apart in the processing direction. After a first sheet has arrived at the downstream rollers, the driving effect thereof on that paper sheet is eliminated, so that the paper sheet is stopped and the end processing of a packet already assembled can take place unobstructedly. A second paper sheet is then supplied by the downstream rollers, until this reaches the downstream pair of rollers. The downstream pair of rollers is then again activated in order to pass the two paper sheets together and deliver them to a holder for subsequent stapling together with a plurality of following paper sheets.
In the case of U.S. Pat. No. 5,289,251, the drive of the two rollers of the downstream pair is completely stopped. In the case of the European patent application 0 613 846, one of the rollers of the downstream pair has a surface with a higher coefficient of friction than the other roller of that pair. The required retention effect is then obtained for the first paper sheet simply by stopping the roller with the higher coefficient of friction. During buffering, the first paper sheet can escape from the path ordinarily taken by the paper sheets. This can also be done constrainedly, by disposing the two pairs of rollers a distance apart less than the length of the paper sheet, so that as the upstream pair of rollers continues to be driven, the trailing part of the first paper sheet will be considerably bent out of the feed path.
Another solution is known from Research Disclosure 1978/16731 and U.S. Pat. No. 4,511,297. Here, before they are delivered to a collecting station, the paper sheets are taken around a relatively large roller, on the outside of which guide paths are provided for the paper sheets. They then arrive at a pair of drive rollers, which can be stopped in order to retain a first paper sheet in order to form a buffer. The first paper sheet will tend to lie against the guide and leave space free at the radial inside for the undisturbed reception of a second following paper sheet. When the second paper sheet has arrived at the pair of rollers, this pair is again operated in order to deliver both sheets to the collecting station.
A disadvantage of these known devices is that to deflect the trailing part of the paper sheet for buffering out of the feed path, they are highly dependent on characteristics of the paper sheet, such as stiffness and flatness and are therefore less suitable for retaining more than one paper sheet, and this limits the time available for carrying out group processing on a bundle of paper sheets preceding the paper sheets for buffering. In particular, a thin and/or curled paper sheet can, during retention, easily jam with its trailing edge in the feed path, and this can lead to transit problems.
Another solution is known from U.S. Pat. No. 5,303,017. The apparatus described therein comprises a driven pair of rollers upstream and a downstream pair of rollers drivable in two directions, with a straight guide path between them. A deflector flap is provided in the guide path and is movable between a position in which the transit path between the two pairs of rollers is free, and a position in which, as considered in the opposite direction to the processing direction, a branch path is created which deflects downwardly. The branch path is defined by guide plates extending curvedly in the downward and upstream direction, while in addition, a third pair of rollers drivable in two directions is disposed in the branch path. When a first paper sheet has been transported through the downstream pair of rollers to an extent such that the trailing edge has passed the deflector flap, this pair of rollers is stopped and the deflector flap is operated into an obliquely downward position. The downstream pair of rollers is then driven in the opposite direction and engagement of the deflector flap on the (initial) trailing edge of the first paper sheet ensures that this paper sheet will follow the deflecting branch path and is engaged by the trailing edge by the third pair of rollers driven in the same direction, with which the first paper sheet must be moved further after the (initial) leading edge has disengaged from the downstream pair of rollers. When the (initial) leading edge of the first paper sheet has passed the deflector flap, the flap is operated to release the main transit path and it is fed to the downstream pair of rollers by means of the upstream pair. The rollers of the third pair are then also driven in the opposite direction in order to force the first paper sheet into the main transit path and to the downstream pair of rollers. Thus, both paper sheets are driven with separate means, arriving jointly, with the second paper sheet somewhat forward, and are engaged by the downstream pair of rollers, in order to be together passed on and delivered to the collecting station situated downstream thereof.
A disadvantage of the apparatus known from U.S. Pat. No. 5,303,017 is that it is fairly complex, necessitates shunting movements over relatively long distances, and thus occupies relatively considerable space, while in addition it is only suitable for retaining just one paper sheet, since only one paper sheet can be fed at a time to the branch path.